You can’t go for a month without seeing a claim that some newdiscoveryhasrewrittenevolutionaryhistory. If headlines are to be believed, phylogeny – the business of drawing family trees between different species – is an etch-a-sketch science. No sooner are family trees drawn before they’re rearranged. It’s easy to rile against these seemingly sensationalist claims, but James Tarver from the University of Bristol has found that the reality is more complex.

Tarver focused on two popular groups of animals – dinosaurs and catarrhines, a group of primates that includes humans, apes and all monkeys from Asia and Africa. Together with Phil Donoghue and Mike Benton, Tarver looked at how the evolutionary trees for these two groups have changed over the last 200 years. They found that the catarrhine tree is far more stable than that of the dinosaurs. For the latter group, claims about new fossils that rewrite evolutionary history (while still arguably hyperbolic) have the ring of truth about them.

For catarrhines, Tarver created a tree using all the species that had been discovered by 2007, including 154 living and 131 fossil ones. He then went back in time to 1760; every ten years, he pruned away species that hadn’t been discovered yet. The goal wasn’t to redraw the tree at each juncture, but to look at how the data had matured over time. Over the years, Tarver worked out how complete the fossil record has been, whether new discoveries cropped up throughout the tree or clustered in specific branches, and whether the tree’s overall shape has shifted or stayed the same.

He found that for catarrhines, claims about evolutionary histories being rewritten are indeed overblown. The shape of the tree that links these animals has been remarkably stable over time, especially within the last century. Scientists had identified most of the key groups early on and when they discovered new species, they tended to uncover hidden twigs on the tree, rather than rearrange entire branches.

But it’s a different story for dinosaurs. As with catarrhines, Tarver took a modern tree and slowly pruned it away in ten-year gaps until 1840. Even though new species are constantly being discovered, the field is apparently “in a state of turmoil”. Our knowledge of dinosaurs is still patchy and new species might hail from entirely unknown lineages. Certain parts of the world are still underrepresented in the fossil record. And while some groups like the flesh-eating theropods have been fairly stable, but others like the giant sauropods are in a state of flux. Tarver concluded that for these ancient reptiles, new discoveries do indeed have the potential to rewrite evolutionary history.

But the difference between theropods and sauropods points to a weakness in Tarver’s analysis: it hinges on a fairly arbitrary choice of groups. By focusing on catarrhines, he narrowed his analysis to a limited group of animals, and one far smaller than the dinosaurs. A less stable picture might have emerged if he had included other primates like New World monkeys or tarsiers. In the same way that theropods were a sea of stability among dinosaurs, perhaps catarrhines are merely a sea of stability among primates as a whole. And even in this group,

There are also important differences between the two groups. Dinosaurs include far more species than catarrhines, and they attract the efforts of far more palaeontologists. Catarrhines have living members representing all corners of the family tree; dinosaurs also have living representatives – birds – but they are a single isolated branch of a mostly extinct tree. Brian Switek, who writes the Laelaps blog, says, “We used living catarrhines to identify particular [lineages], and used this is a template for what to expect from the fossil record. We have no such template for dinosaurs, and so a new fossil can potentially reveal an entirely new [subgroup].”

Switek adds, “The study does not appear to take into account that old discoveries are constantly being reassessed and these studies do sometimes change the shape of the evolutionary tree. W.E. le Gros Clark’s mid-20th century reassessment of the australopithcines moved them out from the “ape” group and into the “human” group, while better material from Ramapithecus showed that it was not as closely related to us as was believed and therefore was booted from the hominin lineage. Focusing on new discoveries alone provides only a narrow view of our changing views of evolutionary trees.” New discoveries also have the potential to shake things up, even among the stable catarrhines, as this year’s discovery of Saadanius demonstrated.

Tarver is aware of these limitations. He writes, “While this confirms media claims that discoveries of new species have substantially altered our perception of the evolutionary history of dinosaurs, this is just as likely to be rejected in light of future discoveries.” To him, the real message of the study is that researchers should try to assess how mature and stable their data are before making bold claims about the evolutionary history of any particular group. “This may provide fewer headlines, but knowledge of evolutionary history that stands the test of time.”

Comments (8)

“[…] researchers should try to assess how mature and stable their data are before making bold claims about the evolutionary history of any particular group”. He is right, but I think that usually the hype is due to the press, rather than the researcher themselves.
Personally right now I can think of just three relatively recent discoveries (or groups of discoveries) which rewrote evolutionary history as I knew it:
– condors being Ciconiiformes rather than vultures
– Annelida being closer to Mollusca than to Arthropoda (I learned this just a year ago reading Dawkins’ “The Ancestor’s Tale”, and after five years of university it was totally unexpected… the definite proof I was waiting for that the professor of animal systematics in my ex-university should really update. Or retire)
– protists being a polyphyletic group made of several kingdoms
But I am sure there are many others

I love the way science writing is approached, scientists are never “cautiously optimistic,” or “curious but with doubts,” or “interested in a new and exciting discovery,” No, scientists only ever experience moments of being “baffled” or having their whole worlds “re-written.” It must be fun being a scientists, only ever experiencing total swings of emotion with absolutely no middle ground.

Thanks for including me in this post, Ed. (And I love the top illustration, by the way. Brilliant.)

Just one note that I did not fully explain in my e-mail; Ramapithecus is no longer a valid genus. As it turned out, the fossils which were originally called Ramapithecus really belonged to a genus of orangutan-like fossil ape from the Siwalik Hills of India and Pakistan called Sivapithecus. It was the recognition that Ramapithecus = Sivapithecus that played a major role in removing it from its place as a potential hominin ancestor.

@ Rhacodactylus – A quick search of the archives shows that I have used the word “cautious” in 11 posts and “caution” in 22. I have used “rewritten” twice, once in this post and once about memories. I have used the word “baffled” twice: in one instance, the creature that was baffled was a clownfish and in the other, it was me!

It seems to me that the two groups might have been chosen because they are the two groups in which new discoveries are most likely to be reported in the popular media as “rewriting evolutionary history”. Since catarrhines is the group that includes human ancestry and it seems that every. single. fossil. bone. ever. found. will always, invariably be said to “rewrite the evolutionary history” of humans (and so far, generally, never does). And dinosaurs, of course, are the prehistoric group that gets the most press in general.

I think that claims that dinosaur phylogenies differ radically from one another, or that they vary much between workers, are overblown, and major areas of consensus have long (or always) been apparent, even for supposedly controversial sauropods. The existence of what we might regard as two ‘opposing schools’ in the study of sauropod phylogeny create the impression that phylogenies have been radically different, but in recent years the two ‘schools’ have been converging in the larger picture and most sauropod phylogenies recover the same basic topology.

I also strongly agree with the fact that comparing dinosaurs and catarrhines is a red herring: sauropod phylogeny might look ‘unstable’ because (until the late 1990s) titanosaurs were regarded by school 1 as closer to diplodocoids than to brachiosaurs (as recovered by school 2)… but if we analyse the history of total primate phylogeny, we see similar problem areas: tarsiers jump on and off the anthropoid branch, for example, and the possible paraphyly of groups like adapiforms and omomyiforms would create for messy ‘competing’ phylogenies.

Re: comment 1 above – condors/New World vultures are best regarded as close to other raptors after all; the idea that they were close to storks was short-lived and based on a dodgy interpretation of genetic data.